A peroxonickel(iii) complex bearing the Me-6-trien ligand, an open-chain analogue of the macrocyclic 12-TMC ligand, was synthesized and characterized by several physical methods, such as UV-vis, resonance Raman, ESI-MS and EPR. Density functional theory calculations provide geometric and electronic configurations of peroxonickel(iii) complexes. The reactivity of the peroxonickel(iii) intermediate was examined in oxidative nucleophilic reactions (e.g., aldehyde deformylation). By comparing the reactivities of the open-chain and macrocyclic peroxonickel(iii) complexes under identical reaction conditions, we were able to demonstrate that the open-chain peroxonickel(iii) complex is much more reactive than the macrocyclic analogue in aldehyde deformylation. This result provides intriguing aspects of the reactivity differences for open-chain vs. macrocyclic systems of metal complexes.